Method and apparatus are known which are capable of measuring the length of a straight line or of an arc. These are used in industry where a machine part either moves back and forth along a linear path, or rotatably about an axis of revolution relative to a fixed part of the machine where the instantaneous position of the movable machinepart is either continuously determined or at predetermined times. In such methods and apparatus the length of a straight line or of an arc is determined by measuring from an arbitrarily defined point at the movable machine part to a fixed predetermined zero point or an arbitrarily determined reference point.
A marking carrier, i.e. for example either a linear scale several meters long or a generally circular scale displaying a plurality of markings is connected with one of the bodies. The markings on the scale are spaced apart in direction of relative movement of the bodies, i.e. linearly or angularly and generally constitute narrow strips or lines extending perpendicularly to the direction of relative movement of the bodies. The other body (the one not provided with the scale) carries a scanning device having a movable measuring sensor adapted to "read" the markings and to generate a signal for each marking it "reads".
The distance of a predetermined point on the body carrying the scanning device from a reference or zero point defined at the other body is the linear or arcuate length to be measured and is determinable by the scanning device. To accurately determine the linear or arcuate length use is made of an index line extending perpendicular to the direction of the relative linear movement and radially through the center of rotation for rotary relative movement. This index line is associated with and determined by the fixed position of a second measuring sensor of the sensing device. It is the spacing of this index line from the zero point of the relative movement which is to be determined.
For highest accuracy and resolution capacity, it has been the practice to provide the linear scale (or the circular scale) with the greatest number of scale markings possible, which makes the width of the markings critical, because it must be made as small as possible, with uniformity of spacing therebetween. The markings must also be as perpendicular as possible to the direction of relative movement of the bodies.
Methods for measurement of an angle or a length of an arc and for measurement of a linear length are known from U.S. Pat. No. 4,449,191 and DE-OS No. 3018527. In the methods of these publications, a rotary movement, which is independent of the relative movement of the two bodies and the angular speed of which is constantly measured, occurs between the marking carrier to be scanned and the measuring sensor of the scanning device. While the methods of the publications employ cheap marking carriers, having comparatively few markings and of relatively great width, and without adhering to uniformity of spacing between the markings and alignment precision, nevertheless afford high measurement accuracy. Here, the marking strips are individually recognized and their spacing precisely measured in a calibration run by measurement of the time distance of the electrical signals generated by the measuring sensor of the scanning device. Through comparison of the calibrated values thus obtained and stored, with the corresponding time distance values obtained during the measuring operation and taking into account the instantaneous speed of rotation, the marking spacing can be determined, its accuracy depending on the accuracy of the time measurement. The length-measuring method of DE-OS No. 3018527 operating on the foregoing principle, has the disadvantage that it requires a complex arrangement for achieving good measurement results. This will be appreciated from the fact that in this publication the linear scale to be scanned is stationary and the relative rotational movement is such that the measuring sensor of the scanning device always moves on a circular path and periodically passes over a predetermined part of the linear scale. Moreover, high measurement accuracy can only be obtained if the rotational speed of the measuring sensor is precisely known and is constant during the measurement operation so that an expensive synchronous motor must be used for the rotational movement. Also a rotary scale rotating with the measuring sensor must be employed which has a plurality of markings scanned by a second measuring sensor fixedly connected to the body carrying the scanning device to measure and monitor the angular speed of the measuring sensor and the circular scale co-rotated therewith. Since, with this method, the markings of the rotating scale disc must likewise be individually identified, their angular spacings must be measured in calibration runs, constantly repeated, and the thus obtained calibration values stored for later comparison with the instantaneously obtained time measured spacing values, requiring thereby expensive data processing components. Beyond that, the radius of the circular path in which the rotating measuring sensor is moved should be as large as possible to minimize measurement error, which otherwise could only be avoided by performing corrective arithmetic operations. In this case, the scanning device is necessarily large and unwieldly so that sealing of internal parts from the surrounding atmosphere becomes difficult if not impossible. Also providing electrical connections for rotating electronic components present a problem especially where the rotating measuring sensor, for obvious reasons, is not to be disturbed.
In the method disclosed in U.S. Pat. No. 4,449,191 it is the circular disc that rotates at high speed so that the above named problems do not occur. However, it is necessary to maintain accurate constant rotational movement. Beyond that, the different marking groups are scanned with two sensors, requiring extensive data-processing. Here also the overall system structure is necessarily large because two groups of markings must be provided on separate rotating scales.